Circuit integrating heterojunction bipolar transistors with pin diodes

ABSTRACT

Generally, and in one form of the invention, a semi-insulating semiconductor substrate 10 is provided having a first surface. An HBT subcollector region 12 of a first conductivity type is implanted in the substrate 10 at the first surface. A PIN diode region 14 of the first conductivity type is then implanted in the substrate 10 at the first surface and spaced from the HBT subcollector region 12. Next, an i-layer 16 is grown over the first surface. Next, an HBT base/PIN diode layer 22 of a second conductivity type is selectively grown on the i-layer 16 over the HBT subcollector region 12 and the PIN diode region 14. Then, an HBT emitter layer 24/26/28 of the first conductivity type is selectively grown over the HBT base/PIN diode layer 22, the HBT emitter layer 24/26/28 having a wider energy bandgap than the HBT base/PIN diode layer 22. Afterwards, an isolation region 30 is implanted at the boundary between the HBT subcollector region 12 and the PIN diode region 14, the isolation region 30 extending down into the substrate 10. Next, the HBT emitter layer 24/26/28 is etched away over the PIN diode region 14. Lastly, conductive contacts 32, 36, 40, 38 and 42 are formed to the HBT emitter layer 24/26/28, the HBT base layer 22, the HBT subcollector region 12, the PIN diode layer 22 and the PIN diode region 14. 
     Other devices, systems and methods are also disclosed.

This is a division of application Ser. No. 07/676,419, filed Mar. 28,1991.

CROSS REFERENCE TO RELATED APPLICATIONS

The following coassigned patent applications are hereby incorporatedherein by reference:

    ______________________________________                                        Serial No. Filing Date                                                                             TI Case No.                                              ______________________________________                                        07/677,019 03/28/91  TI-16012 (filed herewith)                                ______________________________________                                    

FIELD OF THE INVENTION

This invention generally relates to a method of integratingheterojunction bipolar transistors with PIN diodes.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is describedin connection with an integrated transmitter circuit, as an example.

Heretofore, in this field, advanced microwave systems have requireddifferent types of devices for the optimum operation of multifunctioncircuits. For example, the transmitter circuits of communication andradar systems are better addressed with the use of heterojunctionbipolar transistors (HBTs) because of their higher efficiency and higherpower density. On the other hand, receiver circuits of these samesystems prefer the use of field effect transistors (FETs), such as highelectron mobility transistors (HEMTs), to minimize the noise figure andtherefore improve the receiver sensitivity. Other circuit functions inthese systems, such as transmit/receive (T/R) switches and phaseshifting functions may best be accomplished with PIN diodes.

High-speed three terminal devices such as HBTs and heterojunction FETs(HFETs) and microwave diodes such as PIN, IMPATT, mixer, etc. arenormally fabricated by epitaxial growth techniques on insulatingsubstrates. Because of vastly differing epitaxial layer properties foreach one of these devices, each device structure is grown by anepitaxial growth technique that is best suited to the desired propertiesof that layer. Furthermore, the high-speed (or microwave) requirementsplaced on the device forces the growth of each device structure alone onthe substrate to maximize performance. Integration of different types ofmicrowave devices on the same substrate has been attempted in the pastby epitaxial re-growth or ion implantation techniques (see, for example,U.S. patent application Ser. No. 07/560,501, assigned to the sameassignee as the present case).

The integration of two or more heterojunction devices on the samesubstrate cannot be implemented by the method described above. Eachheterojunction growth requires a separate epitaxial growth specificallydesigned to optimize the performance of each heterojunction type. Theintegration of an HBT and HEMT has been attempted in the past (see, forexample, G. Sasaki et al., "Monolithic Integration of HEMTs and HBTs onan InP Substrate and Its Application to OEICs", Int. Electron Dev.Meeting Technical Digest, 1989, p.896) by sequentially growing all ofthe epitaxial layers required for each device at one time, andsubsequently removing any unwanted layers in the area of each deviceduring fabrication. This method is called "stacked-layer method".

Some of the problems posed by the stacked-layer method have been a largenumber of layers required, the need to remove portions of several layersduring processing and a resultant non-planar device. Accordingly,improvements which overcome any or all of the problems are presentlydesirable.

SUMMARY OF THE INVENTION

It is herein recognized that a need exists for a method of integratingheterojunction bipolar transistors with PIN diodes. The presentinvention is directed towards meeting those needs.

The performance, size, volume and cost of such microwave systems can bereduced if all of the subsystem functions can be accomplished with theuse of a common device process technique to integrate all of therelevant advanced devices onto the same substrate. The T/R module of aphased-array radar or a communication system (such as a cellulartelephone), for example, may be fabricated in the form of a singlemonolithic integrated circuit (MMIC) if a process were developed thatwould allow the formation of all of the relevant advanced devices on thesame substrate. At present, this is accomplished by fabricating eachcircuit on separate substrates and then connecting the substrates in amodule.

Generally, and in one form of the invention, a semi-insulatingsemiconductor substrate is provided having a first surface. An HBTsubcollector region of a first conductivity type is implanted in thesubstrate at the first surface. A PIN diode region of a firstconductivity type is then implanted in the substrate at the firstsurface and spaced from the HBT subcollector region. Next, an HBTbase/PIN diode layer of a second conductivity type is selectively grownon the i-layer over the HBT subcollector region and the PIN dioderegion. Then, an HBT emitter layer of the first conductivity type isselectively grown over the HBT base/PIN diode layer, the HBT emitterlayer having a wider energy bandgap than the HBT base/PIN diode layer.Afterwards, an isolation region is implanted at the boundary between theHBT subcollector region and the PIN diode region, the isolation regionextending down into the substrate. Next, the HBT emitter layer is etchedaway over the PIN diode region. Lastly, conductive contacts are formedto the HBT emitter layer, the HBT base layer, the HBT subcollectorregion, the PIN diode layer and the PIN diode region.

An advantage of the invention is that it provides a single process inwhich HBTs and PIN diodes can be fabricated on the same substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further aspects of the invention are illustrated inthe accompanying drawings wherein like elements are denoted by likereference designators and in which:

FIGS. 1A-1G is a cross sectional view showing the steps of constructinga device according to the method of the first preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One example of the epitaxial layer requirements of HBTs, HEMTs, and PINdiodes are shown in Tables 1-3, respectively.

                  TABLE 1                                                         ______________________________________                                        HBT Structure                                                                 No.   Layer        Thickness (μm)                                                                          Doping (cm.sup.-3)                            ______________________________________                                        1     Emitter Contact                                                                            0.15         2-3 × 10.sup.18                         2     Grading      0.05         1 × 10.sup.18                           3     AlGaAs Emitter                                                                             0.10         2 × 10.sup.17                           4     Base         0.10         1-5 × 10.sup.19                         5     Collector    1.00         1 × 10.sup.14 -5 × 10.sup.16      6     Subcollector 1.00         2 × 10.sup.18                           ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        HEMT Structure                                                                No.   Layer          Thickness (μm)                                                                         Doping (cm.sup.-3)                           ______________________________________                                        1     Contact Layer  0.20        3 × 10.sup.18                          2     AlGaAs Donor Layer                                                                           0.05        1 × 10.sup.18                          3     i-Layer (Buffer)                                                                             1.00        1-5 × 10.sup.14                        ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        PIN Diode Structure                                                           No.    Layer      Thickness (μm)                                                                         Doping (cm.sup.-3)                              ______________________________________                                        1      p-Layer    0.10        1-5 × 10.sup.19                           2      i-Layer    1.00        1-5 × 10.sup.14                           3      n-Layer    1.00        2-3 × 10.sup.18                           ______________________________________                                    

It can be seen from Tables 1-3 that the epitaxial layer thicknesses anddoping concentrations of the three types of devices are not directlycompatible with each other. Depending on the frequency of operation, thethicknesses (and possibly the doping concentrations) can change further.The values shown above are for a typical X-band (8 GHz-12 GHz)application.

It is important to note that both the HEMT transistor and the PIN dioderequire a thick (nominally 1.0 μm) i-layer. This layer is essential tothe operation of these two devices, and it must be highly resistive andpure (i.e. low defect density). The HBT structure, although not directlyrequiring the use of such a layer, can accept a layer of similarthickness in its collector. It is the aim of the present invention touse a common i-layer between the three devices and to use modifiedprocessing techniques to enable the growth of all structures on the samewafer without compromising the performance of any of the devices.

A first preferred embodiment of the present invention is a method offabricating HBTs, HEMTs and PIN diodes on the same semiconductorsubstrate. FIGS. 1A-1G show the steps of the method of the firstpreferred embodiment in which the relative dimensions are exaggeratedfor clarity.

In FIG. 1A, a semi-insulating substrate 10, such as GaAs, is provided.Using a suitable mask (not shown), the n⁺ subcollector layer 12 of theHBT and the n⁺ layer 14 of the PIN diode are selectively implanted intothe substrate 10 to a concentration of 2×10¹⁸ atoms/cm². Si is thepreferred ion, but S, Te, Sn, Se or Ge may, for example, also be used.The implanted ions may optionally be activated at this stage or prior tothe epitaxial growth, in the growth chamber (in situ), which follows.

In FIG. 1B, the ion implantation mask (not shown) is removed, thesurface of substrate 10 is cleaned and the layers required for the HEMTdevice are epitaxially deposited across the entire substrate. Theselayers include the i-layer buffer 16, the n-type AlGaAs electron donorlayer 18 and the n⁺ contact layer 20. The donor layer 18 is composed ofa semiconductor material having a wider energy bandgap than the buffer16, preferably AlGaAs. The i-layer buffer 16 and contact layer 20 may bemade of GaAs. Si is the preferred dopant species. Molecular beam epitaxy(MBE) is the preferred deposition method because it has the capacity togrow the HEMT structure with a sharp heterointerface between the layers16 and 18, which is needed for two dimensional electron gas formation,but Metal Organic Chemical Vapor Deposition (MOCVD) can also be used.MBE is also most suited for the growth of the i-layer 16 required forthe HEMT buffer and the PIN structure.

In FIG. 1C, the wafer surface is covered with silicon nitride (notshown). A layer of photoresist (not shown) is then spun on over thesilicon nitride layer and patterned to exposed the regions laying overthe n⁺ buried layers 12 and 14. This pattern is then transferred to thesilicon nitride layer by plasma etching and the remainder of thephotoresist is removed. The silicon nitride layer (which is nowpatterned) is used as an etch mask to remove the n⁺ contact layer 20 andthe AlGaAs channel layer 18 by wet chemical (or reactive ion) etch.Selective re-growth of p-type GaAs base layer 22, n-type emitter layer24, grading layer 26 and n⁺ GaAs emitter contact layer 28 is thenperformed through the silicon nitride mask. Emitter layer 24 is formedwith a semiconductor material having a wider energy bandgap than thebase layer 22, preferably AlGaAs. Grading layer 26 is formed to be asmooth transition in bandgap from the wide bandgap emitter layer 24material (e.g. AlGaAs) at the bottom, to the narrow bandgap emittercontact layer 28 material (e.g. GaAs). Metal Organic Chemical VaporDeposition (MOCVD) is the preferred epitaxial method for this re-growth,but MBE can also be used. Because the thickness of the removed layers isapproximately the same as the re-grown layers, the wafer surfaceplanarity is preserved. The silicon nitride layer is removed at thisstage and this completes the structure growth.

In FIG. 1D, device isolation areas 30 are formed by ion implantation,preferably using oxygen, but gallium, boron or protons can also be used.Photoresist (not shown) is spun on and patterned to define the locationof HBT emitter contact 32 and HEMT source and drain contacts 34.AuGe/Ni/Au metals are evaporated in sequence at thicknesses of 500, 140and 4000 Angstroms, respectively, onto the photoresist and exposedareas. The photoresist is then removed, which lifts off the metal exceptthose portions 32 and 34 that define the HBT emitter and HEMTsource/drain contacts. The contacts 32 and 34 are then alloyed at 430°C. for 1 minute to complete the ohmic contact formation.

As shown in FIG. 1E, the HBT emitter is formed by etching all portionsof epilayers 24, 26 and 28 not masked by emitter contact 32 down to thebase epilayer 22 using a reactive ion etch (RIE in BCl₃, CCl₂ F₂, orCCl₄) or similar anisotropic etch technique. This also removes epilayers24, 26 and 28 in the PIN diode region. The HBT base contacts 36 and thePIN diode anode contact 38 are defined with photoresist and suitablebase metals such as Ti/Pt/Au in thicknesses of 500, 250 and 2000Angstroms, respectively, are sequentially evaporated onto thephotoresist and exposed areas. The photoresist is then removed, whichlifts off the metal except those portions 36 that define the basecontacts and those portions 38 that define the anode contact.

As shown in FIG. 1F, HBT collector contacts 40 and PIN diode cathodecontacts 42 are established by depositing a photoresist mask to definethe collector and cathode contact areas, chemically etching down to theburied layers 12 and 14 in a solution of H₂ SO₄ :H₂ O₂ :H₂ O (1:8:160),and then evaporating AuGe/Ni/Au in sequence at thicknesses of 500, 140and 4000 Angstroms, respectively. RIE can also be used to expose thecollector contact layer 12 at this stage. The evaporation mask is thenremoved which lifts off all metal except in the collector contact 40 andcathode contact 42 areas. The contacts 40 and 42 are then alloyed at430° C. for 1 minute to complete the ohmic contact formation.

In FIG. 1G, the gate area of the HEMT is defined by a photoresist maskand etched down to AlGaAs layer 18 using H₂ SO₄ :H₂ O₂ :H₂ O (1:8:640).Suitable base metals such as Ti/Pt/Au in thicknesses of 500, 250 and2000 Angstroms, respectively, are sequentially evaporated onto thephotoresist and exposed areas. The photoresist is then removed, whichlifts off the metal except that portion 44 that defines the gatecontact.

The remaining process steps required to complete the integrated circuitinvolve such things as forming passive components, forming interconnectmetallization, etc. which are performed as is well known in the art.

A preferred embodiment of the present invention has been described indetail hereinabove. It is to be understood that the scope of theinvention also comprehends embodiments different from that described,yet within the scope of the claims.

For example, a HEMT is used to illustrate the first preferred embodimentmethod, but any type of FET device may be similarly incorporated,including pseudomorphic HEMTs requiring InGaAs layers at theheterointerface. Also, the present invention can provide for thefabrication of pnp type HBTs together with FETs and NIP diodes (a NIPdiode is an upside down PIN diode, which performs the same function in acircuit).

Words of inclusion are to be interpreted as nonexhaustive in consideringthe scope of the invention.

Internal and external connections can be ohmic, capacitive, direct orindirect, via intervening circuits or otherwise. Implementation iscontemplated in fully integrated circuits in silicon, gallium arsenide,or other electronic materials families, but GaAs is highly preferred.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A monolithic microwave integrated circuit (MMIC)comprising:a PIN diode incorporating an i-layer; a heterojunctionbipolar transistor (HBT) incorporating said i-layer as a collector; andan isolation region disposed between said PIN diode and said HBT.
 2. TheMMIC of claim 1 wherein said HBT comprises:a semi-insulatingsemiconductor substrate having a first surface; a subcollector region ofa first conductivity type in said substrate at said first surface; acollector layer on said first surface, said collector layerincorporating said i-layer; a base layer of a second conductivity typeon said collector layer; and an emitter layer of said first conductivitytype on said base layer, said emitter layer having a wider energybandgap than said base layer.
 3. The MMIC of claim 2 wherein saidemitter layer is AlGaAs.
 4. The MMIC of claim 2 wherein said firstconductivity type is n-type and said second conductivity type is p-type.5. The MMIC of claim 2 wherein said collector, base, and emitter layersare epitaxial.